(1) Field of the Invention
This invention relates to a process for preparing 2-chloropropionaldehyde from vinyl chloride, carbon monoxide and hydrogen as raw materials in accordance with the following reaction formula (1): EQU CH.sub.2 .dbd.CHCl+CO+H.sub.2 .fwdarw.CH.sub.3 -CHCl-CHO (1)
2-Chloropropionaldehyde is useful as an intermediate for chemical products, agricultural chemicals, pharmaceutical products, etc.
(2) Description of the Prior Art
It has been known to prepare 2-chloropropionaldehyde from vinyl chloride, carbon monoxide and hydrogen as raw materials, as disclosed by way of example in French Patent No. 1,397,779 and HELVETICA CHIMICA ACTA, 48(5), 1151-1157. All of these processes employ cobalt carbonyl as a catalyst. In French Patent No. 1,397,779 referred to above by way of example, the raw materials were reacted for 90 minutes under conditions of a reaction temperature of 110.degree. C. and a reaction pressure of 200 atm, whereby such productivity as a vinyl chloride conversion of 57.4% and a selectivity of 86.2% toward 2-chloropropionaldehyde was obtained.
However, these conventional processes which employ cobalt carbonyl as a catalyst require cobalt carbonyl in a large amount and a reaction pressure as high as 160-200 atm because the catalytic activity of cobalt is extremely low. Moreover, the reaction is caused to proceed at a reaction temperature of 75.degree.-125.degree. C. for 90-120 minutes. The intended product, 2-chloropropionaldehyde, is a thermally-unstable material. Under the conditions of such a reaction temperature and reaction time, a substantial portion of 2-chloropropionaldehyde is consumed through a consecutive reaction and the reaction yield is thus reduced. Accordingly, these processes have poor reproducibility. Furthermore, hydrogen chloride is byproduced through this consecutive reaction or other side reactions. The byproduction of hydrogen chloride has raised such problems that it causes materials of each reactor to undergo severe corrosion and it reacts with the catalyst, cobalt carbonyl, into cobalt chloride, thereby developing an obstacle to the reutilization of the catalyst.
Some attempts have also been made to apply rhodium catalysts, which have generally been known to exhibit higher activities than cobalt catalysts in oxo reactions, to the above reaction. None of such attempts have however yet proved successful. Reference may for example be made to Tetrahedron Letters, 1969, 22, 1725-1726, in which G. Wilkinson et al conducted the hydroformylation of various unsaturated compounds by using carbonylhydridotris(triphenylphosphine)rhodium [RhH(CO)(PPh.sub.3).sub.3 ] as a catalyst. In the case of vinyl chloride, they reported occurrence of RhCl(CO)(PPh.sub.3).sub.2 and alkenes only. The above attempts are however absolutely silent about such a process that 2-chloropropionaldehyde, the reaction product, and its catalytic component are separated to obtain 2-chloropropionaldehyde with a good yield and at the same time to permit reutilization of the catalyst. As already described above, the reaction product, 2-chloropropionaldehyde, is thermally unstable and the rhodium compound, the catalyst component, also includes some facets for which the rhodium compound may not be said sufficiently stable to heat. For these reasons, the yield of 2-chloropropionaldehyde may not be raised to any sufficient level if its recovery is effected by mere distillation of the reaction mixture. In some instances, it may also be observed that the rhodium catalyst separated and recovered by such a method does not show sufficient activity With the foregoing in view, there has been a long-standing demand for the development of a method for the efficient separation of the reaction product and catalyst.